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B1 -gradient-based MRI using frequency-modulated Rabi-encoded echoes.

Efraín Torres1,2, Taylor Froelich1, Paul Wang1,2

  • 1Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota, Minneapolis, Minnesota, USA.

Magnetic Resonance in Medicine
|September 9, 2021
PubMed
Summary
This summary is machine-generated.

This study introduces frequency-modulated Rabi-encoded echoes (FREE), a new MRI technique that eliminates the need for pulsed field gradient hardware. FREE enables spatial encoding using radiofrequency fields, potentially reducing MRI costs and improving accessibility.

Keywords:
adiabatic full passagefrequency-modulatedportable MRIradiofrequency imagingsilent MRIsurface coil

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Area of Science:

  • Magnetic Resonance Imaging
  • Radiofrequency Physics
  • Medical Imaging Technology

Background:

  • Conventional MRI relies on pulsed magnetic field gradients (B0) for spatial encoding, adding complexity and cost.
  • Existing radiofrequency (RF) imaging methods face limitations due to resonance offset and B1-gradient characteristics.

Purpose of the Study:

  • To develop and evaluate a novel RF imaging method for spatial encoding without B0 gradients.
  • To reduce the expense and increase the accessibility of MRI by eliminating hardware requirements.

Main Methods:

  • Introduced frequency-modulated Rabi-encoded echoes (FREE), an RF imaging technique using adiabatic full passage pulses and a B1 field gradient.
  • Implemented Cartesian phase encoding using FREE in a multi-shot double spin-echo sequence.
  • Conducted theoretical analysis, computer simulations, and experimental comparisons with conventional MRI on human visual cortex.

Main Results:

  • FREE enabled slice-selective 2D imaging of the human brain without a B0 gradient.
  • High resolution was achieved in areas with steep B1 gradients, while nonlinear B1 fields caused image distortions.
  • Minimal signal loss occurred due to B1 nonlinearities and resonance offset.

Conclusions:

  • Frequency-modulated Rabi-encoded echoes (FREE) is a viable method for MRI spatial encoding without B0 gradients.
  • Potential for future correction of image distortions using B1 and B0 maps.
  • This technique could lead to more accessible and cost-effective MRI.